Note: Descriptions are shown in the official language in which they were submitted.
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A Method of Coding and Decoding Audio Signals
FIELD OF THE INVENTION
The present invention refers to coding and decoding methods
for audio signals and especially to the use of different
window functions and synthesis window functions, respecti-
vely, in dependence upon the audio signal to be coded or the
coded audio signal.
DESCRIPTION OF THE PRIOR ART
Modern audio coding methods, such as the methods according
to the standard MPEG layer 3 or according to the standard
MPEG2-NBC, which is in the standardization phase, produce
blocks of encoded audio signals.. As will already be evident
from the name (NBC - non backward compatible), the above-
mentioned standard MPEG2-NBC, which is in the standardiza-
tion phase, need not be backward compatible. The present
invention now refers to a further development within the
framework of the future standard MPEG2-NBC.
Coding of an audio signal present in analog form, which
results in most ,cases in substantial compression of the
data, first requires sampling of said audio signal. In the
description of the present invention, a sampling frequency
of 48 kHz is used. This is, however, an arbitrary choice; it
is also possible to use other sampling frequencies which are
normally used for audio signa:Ls. After time sampling, the
audio signal is present in the form of a time-discrete audio
- signal consisting of an equidistant sequence of individual
sampling values. The time interval between one sampling
value and the next is equal to the inverse of the sampling
frequency used for sampling. According to the sampling
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theorem, the bandwidth of the analog signal must be re-
stricted to half the sampling f~__°equency in order to recon-
struct unequivocally the analog signal from the sampled
signal.
As has already been mentioned, audio coding methods, such as
MPEG2-NHC, produce coded blocks of data. From the hitherto
so to speak endless stream of time-discrete audio signals,
successive blocks are produced by windowing with overlapping
window functions. The window funcaion can, for example, be a
sine window. Those skilled in ths~ art know, however, a large
number of other possible window functions. When normal cod-
ing is carried out, the window length for MPEG2-NBC is 2048
sampling values.
The time length of a window fun~~tion results from the pro-
duct of the 2048 sampling values and the inverse of the
sampling frequency; for the present example, a window length
of 42.67 ms would be obtained, if the individual window
functions did not overlap. MPEG2-NBC uses, however, a 50%
overlap and a subsequent MDCT (MDCT - modified discrete
cosine transform), and this results in blocks with 1024 fre-
quency values per block. In view of the fact that the in-
dividual window functions overlap by 50%, it would be un-
necessary to produce 2048 frequEancy values for each window
function, since the resultant data would then have a 50%
redundancy. Hence, whenever a block of data, i.e. frequency
values, is produced, two neighbouring window functions take
part, which overlap and add. To sum up, it can be said that
a window function has a time length of 2048 sampling values
multiplied by the inverse of the sampling frequency, whereas
a block has 1024 frequency values which are determined by
overlap and add making use of taro neighbouring window func-
tions. The frequency values supp:Lied by the MDCT must subse-
quently be quantized for digital further processing.
This quantization adds to the time-discrete audio signal a
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disturbance in the frequency range whose permitted magnitude
was calculated in a psychoacoustic model of the encoder. In
view of the fact that, due to the windowing carried out, the
time resolution is predeterminE:d, the quantization distur-
bance smears so to speak over i:.he whole length of the time
window.
The mutual distance between the 1024 frequency values for
each block is equal to the quotient of half the sampling
frequency and the number of sampling values. In order to
guarantee that the energy conservation law is fulfilled,
each spectral coefficient, i.e. each frequency value, has a
bandwidth that corresponds to t:he above-mentioned quotient.
The time resolution is equal to the inverse of the frequency
resolution, i.e. equal to the quotient of the number of
sampling values and the sampling frequency. Expressed in
numerical values, at a sampling frequency of 48 kHz this
time resolution is 1024 - 1/48000 s - 21.33 ms. A quanti-
zation disturbance will, however, "smear" veer a whole win-
dow, i.e. over a time period of 2 ' 21.33 ms = 42.66 ms.
In the case of audio signals signals varying strongly with
time and having transient components, this poor time resolu-
tion may have the effect that the quantization disturbance
distributed over this block becomes audible as a pre- or
post-echo when the audio signals will be decoded later on.
In order to prevent the pre-echo or postecho disturbance,
the overlapping long window functions, i.e. window functions
with 2048 sampling values, are replaced by a plurality of
overlapping short window functions in windows with signal
regions that vary strongly with time. In order to guarantee
correct coding, i.e. especially a correct overlap and add,
with the shorter window functions subsequent to the longer
window functions, the coding with shorter window functions
must be initiated by a so-cal7.ed start window sequence and
terminated a so-called stop window sequence, since a block
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with frequency values is formed. by the cooperation of two
neighbouring overlapping window functions.
In the NBC encoder mentioned above, eight 50% overlapping
window functions, which each have a length of 256 sampling
values, are used. The time resolution is improved to 128
1/48000 s - 2.67 ms in this way, whereby pre-echoes are
essentially avoided. Here, too, there is a distribution of
the quantization disturbance over a time domain which is
twice as long, i.e. 2 ~ 2.67 ms =- 5.34 ms.
In Fig. 2, a known block sequence, i.e. a group of individ-
ual window function sequences, is shown which is obtained
when switching over to a window function of shorter length
is effected. The block sequence shown in Fig. 2 will be ex-
plained hereinbelow from the left to the right, whereby the
horizontal line can be a section of the time axis. Prior to
describing the block sequence, reference is made to the fact
that, in order to make things clearer, the above-mentioned
sine curve of the window function is shown in simplified
form by straight lines in Fig. and in all other figures.
In actual fact, special window functions, such as those
described in the standard MPEG1 (ISO/11172-3) are used. An
ascending straight line thus corresponds to the first half
of e.g. a sine window functi-on, whereas a descending
straight line corresponds to the second half of a sine win-
dow function.
A descending line lOb in Fig. 2 represents the second half
of a window function 10 for signals varying weakly with
time, i.e. the window function with e.g. 2048 sampling
values. From said figure, it can be seen that a so-called
long block 12 is formed by the overlap of the second half
lOb of the long window function 10 and of an ascending line
14a of a start window sequence 14, i.e. a sequence of window
functions including in addition to the ascending line 14a a
constant part 14b and a descending line 14c, which is the
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same as the second half 16b of a short window function 16
for signals varying strongly with time. The start window se-
quence 14 is followed by eight short window functions 16,
i.e. window functions for signals varying strongly with
time, which are necessary for windowing transient audio sig-
nals so as to minimize the audibility of the quantization
disturbances, as has already been described hereinbefore.
These 50% overlapping short window functions, each compris-
ing 256 sampling values, form a short block 13, which com-
prises 128 frequency values. The eight short window func-
tions 16 are again followed by a stop window sequence 18
comprising first the first half 18a of the short window
function 16, and then a constant part 18b and a descending
line 18c, which is the same as the second half lOb of the
long window function 10, i.e. the window function for sig-
nals varying weakly with time.
Due to the descending line 18c as well as the first half 10a
of the long window function 10, it is now possible to
produce correct frequency valuea also in block 12, extreme
right in Fig. 6. The start window sequence 14 and the stop
window sequence 18 therefore guarantee that, independently
of the switching over of window functions, correct frequency
values for a long block 12 and a short block 13,
respectively, can be produced in areas with signals varying
weakly with time and in areas with signals varying strongly
with time. In all figures, the section of the time axis
shown is subdivided into blocks of 1024 sampling values,
each of said blocks being subdivided into eight units,
whereby 128 sampling values or ~'requency values are obtained
for one unit.
One disadvantage of the use of short window functions 16 for
- forming short blocks 13 is the fact that the coding effi-
ciency of said short window functions is worse than that of
long window functions; hence, it is attempted to avoid
switching over from long window functions to short window
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functions as far as possible. In this connection, reference
is made to the fact that side information, i.e. additional
information, must be transmitted in addition to each data
block transmitted, said side information indicating e.g. the
window function which has been used for coding in connection
with a specific block.
The start window sequence 14 shown in Fig. 2 and the stop
window sequence 18 are mirror images of one another and they
have the same length. A start-short-stop block sequence ac-
cording to the prior art is comparatively long, the number
of short window functions 16 being always fixed at eight
short blocks so as to keep to the block raster of 1024. In
an area 20, transients may occur. It is therefore not pos-
sible to switch less than eight short window functions, even
if the Length of the transient area is shorter than area 20.
Furthermore, due to the constant parts 14b, 18b, the start-
short-stop-block sequence in Fig. 2 is comparatively long,
and in the case of transients occurring at a time interval
of some length it is therefore not possible to switch over
again to a long window function between the transients.
Hence, it is necessary to use more short window functions
than necessary, whereby the coding efficiency is unneces-
sarily impaired. If the transient additionally occurs at the
margins of the area in question, it will often be necessary
to insert a second window sequence of eight short windows
into the block sequence according to Fig. 6 in order to ful-
ly include said block sequence.
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EP A 559383 discloses a method for coding audio signals
based on a perceptional model. Using perceptional
principles. Two basic window lenghts are provided. The
window having the first length windows a number of 1024
input samples, whereas the window having the second length
windows a number of 256 input samples. Short windows are
associated in sets of four to represent as much spectral
data as a large window. In order t.o make the transition from
large to short windows and vice versa two more types of
windows are used. The first window is a START window that
makes the transition from large to short windows. Another
one is a STOP window that makes the opposite transition.
Thus, possible block sequences include: the start window and
the stop window; and a start window, a variable number of
sets of four short windows and a stop window. The START
window, the STOP window and the :Long and short windows are
stored in a window memory.
WO 91 16769 A discloses encoders and decoders for audio
coding which are capable to adapt analysis windows and
transform methods to process signals with respect to
perceptual characteristics. A frame controller chooses an
analysis window with the appropriate shape and length
depending on the to be coded signals. The frame controller
is able to carry out two frame control modes: a fixed-frame
alignment mode; and a dynamic-frame alignment mode. The
fixed-frame alignment mode controls the frame length of
windows in a window sequence such that transient signals are
windowed by smaller windows and that a constant frame length
is maintained from the beginning of the windowing with
smaller windows to the end thereof. The dynamic-frame
alignment mode allows different: block lengths for the
processing of transient signals. The different frame lengths
are set such that only a minimum number of short windows are
necessary to code transient signals. Different transition
window sequences are dynamically calculated during
processing of time samples.
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SUMMARY OF THE INVENTION
It is the object of the present invention to provide a cod-
ing method and a decoding method for audio signals which
minimize the use of short blocks without causing a deteri-
oration of the coded/decoded audio signals originating from
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quantization disturbances.
In accordance with a first a:~pect of the invention, this
object is achieved by a method of encoding time-discrete
audio signals, comprising the step of weighting the
time-discrete audio signal by means of window functions
overlapping each other so as to form blocks, said window
functions producing blocks of a first length for signals
varying weakly with time and blocks of a second length for
signals varying strongly with time, a start window sequence
being selected for a transition from windowing with blocks
of said first length to windowing with blocks of said second
length and a stop window sequence being selected for an
opposite transition, wherein 'the start window sequence is
selected from at least two different start window sequences
and the stop window sequence is selected from at least two
different stop window sequences.
In accordance with a second aspect of the invention, this
object is achieved by a method of encoding time-discrete
audio signals, comprising the step of weighting the
time-discrete audio signal by means of window functions
overlapping each other so as to form blocks, said window
functions producing blocks of a first length for signals
varying weakly with time and :blocks of a second length for
signals varying strongly with time, a start window sequence
being selected for a transition from windowing with blocks
of said first length to windowing with blocks of said second
length and a stop window sequence being selected for an
opposite transition, said window functions comprising a
combined stop start window sequence which produces a block
of the first length and which is situated between window-
sequences producing blocks of the second length.
In accordance with a third aspect of the invention, this
object is achieved by a method of decoding time-discrete
audio signals encoded in bloclts, said method comprising the
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following steps:
reading of side information wit: which said encoded blocks
are provided, said side information referring to the window
function which has been used in ~~onnection with the block in
question and to the transformation which has been used with
the block in question;
selecting an inverse transformai=ion and a synthesis window
function as a reaction to the side information read; and
re-transforming and windowing with the selected inverse
transformation and the selected ;synthesis window function.
The present invention is based on the finding that the cod-
ing efficiency can be increased considerably by providing a
plurality of different start and stop window sequences of
different lengths; in this connection, it is possible to
selectively choose, depending on the time-discrete audio
signals to be coded, a single start or stop window sequence
for maximum avoidance of pre- and post-echoes caused by the
quantization following the trans:Eormation.
Further a new combined STOP-STAF;T window sequence producing
a long block permits a clearly reduced repetition time for
switching short blocks compared. to Fig. 2. This combined
STOP-START window sequence ends a window sequence consisting
of short window functions and serves as start window
function for a further window sequence of short window
functions.
It follows that the methods according to the present inven-
tion no longer rigidly demand fixed transformation lengths
of e.g. 1024 values for the MDCT, but they permit the use of
different transformation and retransformation lengths.
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On the basis of the now admiti~ed different transformation
lengths as well as on the basis of a plurality of start and
stop window sequences that can be chosen selectively, the
methods for encoding and decoding according to the present
invention can react flexibly and appropriately to areas of
the time-discrete audio signal in which block lengths of
I024 values are too long.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred embodiments of the present invention will be ex-
plained hereinbelow more precisely, making reference to the
drawings enclosed, in which:
Fig. 1 shows possible window sequences in the MPEG2-NBC
coder/decoder;
Fig. 2 shows a known window sequence for switching over
from long window functions to short window
functions and vice veraa in the NBC coder.
Fig. 3 shows an example of a window sequence according to
the present invention;
Fig. 4 and Fig. 5 show exams>les of window sequences ac-
cording to the present invention including a
minimum number of short blocks; and
Fig. 6 shows an example of a block sequence according to
the present invention for reducing the repetition
time of the method shown in Fig. 2; and
Fig. 7 shows an example of a block sequence according to
the present invention for reducing the repetition
time between two transient events.
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DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
Fig. 1 shows possible window sequences which can be used in
the method according to the presE~nt invention for coding and
decoding time-discrete audio signals according to the MPEG2-
NBC standard. In the first column of Fig. 1, the number of
the individual window sequences is shown; in this connec-
tion, reference is made to the fact that the seven possible
window sequences can be coded by three bits; in comparison
with the former standard MPEG layer 3, this only means one
bit of additional side information per channel, since said
standard already needs two bits for characterizing the win-
dow function used for a block.
The window sequence No. 4 is not occupied and is considered
to be reserved.
In the second column of Fig. 7., the name of the window
sequence shown in the fourth column appears, whereas the
third column shows the number of spectral coefficients for
each window sequence. As has a7Lready been mentioned, the
window sequence by means of which pre- and post-echoes can
be avoided best is selected from the various extended window
sequences in the encoder. In accordance with the present
invention, this can be done in a much more selective manner
than in the case of conventional encoders. The length of the
MDCT required and thus the ma:~cimum number of quantized
frequency lines to be transmitted is 1024 for each sequence.
After windowing, an MDCT having the length determined is
executed. In accordance with a preferred embodiment of the
present invention, a plurality of MDCTs having different
lengths (i.e. numbers of spectral coefficients) can be
called, the frequency values of said MDCTs being then juxta-
posed. The fifth sequence in Fig. 1 requires an MDCT having
a length of 640 and, in addition, three MDCTs each having a
length of 128 spectral coefficients, whereby the shown num-
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ber of 1024 spectral coefficients is obtained. It follows
that the present invention permits varying transformation
lengths and, consequently, also varying block lengths.
In the first line having number 0 in Fig. 1, the long window
function 10, which is named ONLY_LONG and which has already
been described, is shown, said window function 10 being com-
posed of the ascending first half 10a and of the descending
second half 10b. This window function 10 is the window func-
tion which normally is to be u~:ed for audio signals varying
weakly with time. In this conne~~tion, it is pointed out that
the expression window sequences actually stands for a se-
quence of window functions. Although the window function 10
consists of only one window function, it will be referred to
as window sequence ONLY-LONG 10 hereinbelow for reasons of
consistency. Hence, a window function within the meaning of
the present application can comprise only one window func-
tion or also a sequence of window functions.
If this window sequence ONLY-:LONG 10 is not suitable for
interference-free coding of audio signals, since said audio
signals vary strongly with time, a change-over to the short
window function 16 must be carried out for the purpose of
coding. The window sequence EIGHT-SHORT 32 represents a
sequence of eight short window functions for interference-
free coding of audio signals varying rapidly with time.
In accordance with a preferred embodiment according to the
present invention, the short window functions 16 as well as
the long window functions 10 each overlap by 50%. In order
to achieve perfect coding of the overlapping window func-
tions, a sequence of short window functions can be initiated
by means of a start window sequence, such as the start win-
dow sequence LONG-START 36 o:r the start window sequence
SHORT_START 38. The start window sequence LONG-START 36
corresponds to the start window sequence 14, which has been
described in connection with Fig. 2.
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The start window sequence SHOR'.~ START 38 consists of the
first half 10a of the window function 10 and of three short
window functions 16, a constant window segment 19, the
length of which corresponds to 64 sampling values followed
by the second half 16b of a sho t window function 16 being
located at the transition from the first half 10a of the
window function 10 to the overlapping three short window
functions 16. The start window sequence SHORT START 38
comprises three short window functions 16 that are already
integrated therein.
The method according to the present invention permits the
use of two stop window sequences LONG STOP 40 and SHORT STOP
42. The stop window sequence LONG STOP 40 is identical to
the stop window sequence 18 shown in Fig. 2. The stop window
sequence SHORT STOP 42 is analogous to the start window se-
quence SHORT START 38.
Line 6 in Fig. 1 shows a combined STOP-START window sequence
41 comprising a first part which corresponds to the first
half 16a of a short window function, a constant second part
with a length of 7 x 128 sampling values as well as a third
part which corresponds to the :second half 16b of a short
window function. The combined STOP-START window sequence 41
can be switched between short wirsdow functions which produce
short blocks. The STOP-START window sequence 41 produces a
long block as it is shown in column 3 of Fig. 1.
In principle, all possible combinations, i.e. block se-
quences, consisting of a start window sequence 36, 38, an
arbitrary number (n, m) of the window sequence 32 and a stop
window sequence 40 can be used. In accordance with a
preferred embodiment of the present invention, only the
following combinations are, however, used. Despite the
varying positions of the short window functions the
preferred raster including e.g. 1024 spectral coefficients
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is always maintained. The combinations used are therefore
the following ones:
startwin- window se- stop window
dow equence quence sequence
s
SHORTSTART 38 SHORT STOP42
SHORTSTART 38 n*EIGHT SHORT 32 SHORT STOP42
LONG START 36 n*EIGHT SHORT 32 LONG STOP40
LONG START 36 n*EIGHT SHORT 32, STOP START 41,
m*EIGHT SHORT 32 LONG STOP40
SHORTSTART 38 LONG STOP40
SHORTSTART 38 n*EIGHT SHORT 32 LONG STOP40
LONG START 36 SHORT STOP42
LONG START 36 n*EIGHT SHORT 32 SHORT STOP42
SHORTSTART 38 STOP SHORT STOP42
START
41
Table 1
In the following, some exemplary block sequences will be
described, which can be formed by the window sequences shown
in Fig. 1.
Fig. 2 shows the previously us<~d block sequence which has
already been described. This ca:n be composed of the window
sequences LONG_START 36, EIGHT_,SHORT 32 and LONG_STOP 40.
Reference numeral 20 again indicates the area in which tran-
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sients may exist in the audio signal.
The present invention, however, does not demand a window se-
quence which is symmetrical with regard to the block bound-
ary, as shown e.g. in Fig. 3. Fig. 3 is a combination of the
window shapes SHORT_START 38 and SHORT STOP 42. This block
sequence now comprises six short blocks 13. It follows that,
in comparison with the conventional method shown in Fig. 2,
it is possible to correctly encode a transient event in six
short blocks 13 with six short window functions, instead of
encoding it in eight blocks wii=h eight short window func-
tions. When the known method according to Fig. 2 is used,
1024 frequency coefficients are obtained for start and stop
as well as for each of the eight. short windows, whereby the
block raster is always observed.
Unlike this the block sequence according to Fig. 3 permits
now the windowing of a transient event in the area 20
comprising only 2 x 1024 spectral coefficients, whereby the
block raster is always observed <~s well.
In Fig. 3, it can be seen that the short window sequences
cover areas for transients concerning the block raster which
overlap regarding Fig. 2, wlnereby transients can be
incorporated much more specifically. It follows that, ac-
cording to the present invention, it is possible to cover
any transient by a maximum of six or eight short blocks. In
connection with the known method according to Fig. 2, it was
often necessary to use two successive window sequences each
consisting of eight short blocks..
Furthermore, in comparison with the known method according
to Fig. 2, the block sequence in Fig. 3 has a considerably
reduced repetition time, i.e. between closely spaced
transients it is possible to switch back to long blocks far
faster than in the case of Fig. 2.
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By introducing the combined STOP-START window sequence 41
also repetition time 50 of they known method for switching
blocks which is described by Fig. 2 can be reduced
considerably. This is shown in Fig. 6 and is described later
on.
The block sequence start-stop :in Fig. 3 is followed by the
long window sequence ONLY-LONG 10, which is used for encod-
ing signals that vary weakly with time. At this point, ref-
erence is made to the fact that the terminology "varying
weakly with time" and "varying strongly with time" are only
relative terminologies; in accordance with a special embodi-
ment, a so-called transient threshold is predetermined at
which the window function used is changed. This transient
threshold will depend on the window length of the long win-
dow function.
Fig. 4 and 5 represent a further improvement of the coding
efficiency. Fig. 4 is essentially composed of the start win-
dow sequence SHORT-START 36 and the stop window sequence
LONG STOP 40. In this example, the number of short windows
is only three blocks. It follows that short transient events
can be encoded with only three short window functions 16. In
comparison with Fig. 2, this represents a considerable
reduction of the short window :Functions, whereby the number
of short window functions and, consequently, also the number
of short blocks 13 is minimized. Furthermore, the block se-
quence shown in Fig. 2 corresponds to the length of two long
blocks 12 so as to be able to return to the long window
function, whereas in Fig. 4 switching over is, like in Fig.
3, finished after the length of one block.
Another possible variation is shown in Fig. 5. Because of a
transient area in the time-discrete audio signal, switching
over from a long window function to short window functions
16 is effected. This is initiated by the start window se-
quence LONG_START 36 and finished by the stop window
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function SHORT-STOP 42 which already includes three short
window functions 16. Hence, Fig,. 4 and Fig. 5 clearly show
that a large number of transient events in the time-discrete
audio signal is now covered by orally three short window func-
tions 16, i.e. three short blocks 13. As a result, the
coding efficiency can be increased still further, as has
already been mentioned.
As the block sequences according to Figs. 4 and 5 cover
different time areas regardinc; the block raster, many
transient areas can be covered by them. According to the
prior art of Fig. 2 it is only possible to window transient
events symmetrically to the limit of the block by means of
short window functions, whereas the block sequence of Fig. 4
can window transient events in the first half of a long
block and the block sequence of Fig. 5 can window transient
events in the second half of a long block.
As already mentioned, Fig. 6 shows a block sequence which
achieves a reduced repetition time 50 by introducing the
combined STOP_START window sequence for the known method
according to Fig. 2. The start window sequence LONG START 36
is followed by the window sequence EIGHT SHORT 32. Same is
finished by the window sequence STOP START 41 which at the
same time serves as start window sequence for a further
window sequence EIGHT_SHORT fini~;hed then by the stop window
sequence LONG STOP.
Fig. 7 shows a block sequence which enables a minimum number
of short blocks for a transient event as well as a minimum
switching time. The window function ONLY_LONG 10 for signals
slowly varying in time is followed by the start window
sequence SHORT-START 38 which already comprises three short
window functions. The combined windaw sequence STOP START 41
finishes the sequence of short window functions and at the
same time initiates a further sequence of three short window
functions which are included in the stop window sequence
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SHORT STOP 42.
To sum up, it can be stated that all block sequences shown
in Fig. 2 to 7 can be produced by making use of the window
sequences shown in Fig. 1. As has already been stated in
connection with table 1, it is also possible to produce
block sequences with an arbitrary number of additional win-
dow sequences EIGHT_SHORT 32. Coding of unsuitable signals
may necessitate this. Due to its flexibility, the "syntax"
for the formation of block sequences shown in Fig. 1 permits
this easily. The element 4 which is not yet occupied can be
used for extensions which have not yet been implemented.
As has already been mentioned, seven different window se-
quences are specified in Fig. 1; said window sequences can
be identified by three bits in the bit stream. If the area
to be transmitted contains short blocks, these are grouped
together because the items of ride information would be too
large if they were not combined. How many blocks are grouped
together in each individual case is determined algorithmi-
cally. This information is then also transmitted to the de-
coder by means of additional side information bits. Within a
group of short blocks, the spectrum is then re-sorted. As
has already been mentioned at i:he very beginning, the spec-
tral values, i.e. the frequency values, are quantized taking
into account the psychoacoustically permitted disturbance.
This quantization is, however, no longer influenced by the
method according to the present invention.
The decoder, which carries out a method of decoding the en-
coded audio signal, annuls all the signal modifications
which have been carried out in the encoder. For this pur-
pose, the frequency values must first be resealed and the
quantization of said frequency values must be annulled, i.e.
they must be requantized. Subsequently, they must be re-
sorted so as to annul the specltral sorting. Following this,
retransformation can be carried out making use of the in-
CA 02260033 1999-O1-08
WO 98/02971 PCTIEP96105145
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verse MDCTs (IMDCTs) specified by the side information.
After selection of one of the synthesis window functions
associated with the window funcaion used and subsequent
synthesis windowing as well as the taking into account of
the overlap and add, the decoded discrete sampling values of
the audio signal are obtained again.
